Planar optical waveguides for optical panel having gradient refractive index core

ABSTRACT

An optical panel is disclosed. A plurality of stacked planar optical waveguides are used to guide light from an inlet face to an outlet face of an optical panel. Each of the optical waveguides comprises a planar sheet of core material having a central plane. The core material has an index of refraction which decreases as the distance from the central plane increases. The decrease in the index of refraction occurs gradually and continuously.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/543,380, filed Apr. 5, 2000 now U.S. Pat. No. 6,307,995.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.DE-AC02-98CH10886 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of display devices.In particular, the present invention relates to an optical panel havinga plurality of stacked planar optical waveguides for guiding light froman inlet face to an outlet face of the optical panel. More specifically,the present invention relates to a plurality of stacked planar opticalwaveguides for an optical panel, the planar optical waveguidescomprising a core material having a gradient refractive index.

2. Description of the Background

Optical screens typically use cathode ray tubes (CRTs) for projectingimages onto the screen. The standard screen has a width to height ratioof 4:3 with 525 vertical lines of resolution. An electron beam isscanned both horizontally and vertically across the screen to form anumber of pixels which collectively form the image.

Conventional cathode ray tubes have a practical limit in size, and arerelatively deep to accommodate the required electron gun. Larger screensare available which typically include various forms of image projection.However, such screens have various viewing shortcomings includinglimited viewing angle, resolution, brightness, and contrast, and suchscreens are typically relatively cumbersome in weight and shape.Furthermore, it is desirable for screens of any size to appear black inorder to improve viewing contrast. However, it is impossible for directview CRTs to actually be black because they utilize phosphors to formimages, and those phosphors are non-black.

Optical panels may be made by stacking planar optical waveguides, eachwaveguide having a first end and a second end, wherein an outlet face isdefined by the plurality of first ends, and wherein an inlet face isdefined by the plurality of second ends. Such a panel may be thin in itsdepth compared to its height and width, and the cladding of thewaveguides may be made black to increase the black surface area. Asshown in FIG. 4, these optical panels typically comprise planar opticalwaveguides 10 a of the type which include discrete cladding layers 82directly adjacent to and surrounding core layers 80. The cladding layers82 have an index of refraction which is discretely lower than that ofthe core layers 80 and thus enables transmission of light 22 by internalreflection. This results in discrete reflections, or bounces, of thelight 22 at interfaces 95 between the cladding layers 82 and core layers80. This optical waveguide configuration is of the type which will bereferred to hereinafter as “step index cladding”.

However, optical waveguides of the step index cladding type have atleast two significant drawbacks. First, a small loss of light takesplace at each bounce at the interface 95 between the core layer 80 andsurrounding cladding layers 82. Although the loss of light at eachbounce within the optical waveguide is extremely small, a light ray mayundergo a large number of bounces as it traverses the core layer.Optimally, it is desired to have the core layer thickness to be as smallas possible to achieve higher resolutions. But, as the core layerthickness decreases, the number of bounces the light ray must endureincreases. Therefore, the amount of light loss that occurs in opticalpanels (and in particular, higher resolution optical panels), becomes asignificant detriment to the overall efficiency and performance of theoptical panel, as well as the quality (e.g. brightness, sharpness, etc.. . . ) of the image.

FIG. 5 illustrates the second significant drawback of using opticalwaveguides of step index cladding type. When light 22 entering corelayer 80 comprises two or more different wavelengths, a phenomenon knownas chromatic dispersion results. As shown in the figure, light 22comprising two different wavelengths, even entering the core layer 80 atthe same angle, will be displaced when exiting the core layer 80resulting in two corresponding light rays 22 a, 22 b. These light rays22 a, 22 b exit the outlet face of the optical panel at slightlydifferent exit angles resulting in poor color quality of the image. Thismeans that the exit angle of the light at the outlet face of the opticalpanel is dependent on the wavelength, or color, of the components of theinput light. As can be envisioned, this phenomenon is furtherexaggerated when the light path that a light ray follows through thecore layer 80 increases. For example, the chromatic dispersion effectincreases as the core layer 80 becomes longer (i.e. in the directionthat the overall light travels therethrough) for larger optical panels.Thus, the chromatic dispersion that occurs in optical panels usingoptical waveguides of step index cladding type is another significantdetriment to the performance of the optical panel, as well as thequality (e.g. color, sharpness, etc. . . . ) of the image.

Therefore, the need exists for an optical panel which possesses theadvantages corresponding to the use of stacked optical waveguides, butwhich does not suffer from the decrease in efficiency, performance andquality resulting from the light loss from the discreet bounces that thelight undergoes in the optical waveguides of step index cladding type,nor suffer from the deleterious effects of chromatic dispersion whenusing optical waveguides of step index cladding type.

SUMMARY OF THE INVENTION

The present invention is directed to a plurality of stacked planaroptical waveguides for an optical panel. Each optical waveguidecomprises a planar sheet of core material having a central plane. Thecore material has an index of refraction which decreases as the distancefrom the central plane increases.

The present invention is also directed to an optical panel whichincludes a plurality of stacked optical waveguides. Each opticalwaveguide has a first end and a second end. An outlet face is defined bythe plurality of first ends and an inlet face is defined by theplurality of second ends. Each optical waveguide comprises a planarsheet of core material having a central plane. The core material has anindex of refraction which decreases as the distance from the centralplane increases.

The present invention solves problems experienced in the prior art, suchas the decrease in efficiency, performance and quality resulting fromthe light loss from the discreet bounces that the light undergoes in theoptical waveguides of step index cladding type, and the adverse affectsof chromatic dispersion when using optical waveguides of step indexcladding type, by providing a plurality of planar optical waveguides foran optical panel, the planar optical waveguides comprising a corematerial having a gradient refractive index. The present invention alsoretains the advantages which correspond to a stacked optical waveguidepanel, such as improved contrast and minimized depth.

Those and other advantages and benefits of the present invention willbecome apparent from the detailed description of the inventionhereinbelow.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For the present invention to be clearly understood and readilypracticed, the present invention will be described in conjunction withthe following figures, wherein:

FIG. 1 is an isometric view schematic illustrating an optical panel;

FIG. 2 is a side view cross sectional schematic of an optical panel;

FIG. 3 is a schematic illustrating a horizontal and vertical crosssection of an optical panel using planar optical waveguides that utilizestep index cladding;

FIG. 4 is a side view cross sectional schematic of a planar opticalwaveguide that utilizes step index cladding;

FIG. 5 is a side view cross sectional schematic of the planar opticalwaveguide shown in FIG. 4 illustrating chromatic dispersion of the lighttraveling therethrough;

FIG. 6 is a side view cross sectional schematic of a planar opticalwaveguide that utilizes a gradient refractive index core, in accordancewith a preferred embodiment of the present invention;

FIG. 7 is a side view cross sectional schematic of the planar opticalwaveguide shown in FIG. 6 illustrating the reduction or elimination ofchromatic dispersion, in accordance with a preferred embodiment of thepresent invention;

FIG. 8 is a side view cross sectional schematic of an optical panelutlizing a light diffuser at the outlet face; and

FIG. 9 is a side view cross sectional schematic of a planar opticalwaveguide that utilizes a gradient refractive index core and a gradientblack diffuser, in accordance with a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, many other elements found in a typical opticaldisplay panel. Those of ordinary skill in the art will recognize thatother elements are desirable and/or required in order to implement thepresent invention. However, because such elements are well known in theart, and because they do not facilitate a better understanding of thepresent invention, a discussion of such elements is not provided herein.

FIG. 1 is an isometric view schematic illustrating an optical panel 10.The optical panel 10 includes a plurality of waveguides 10 a, whereinone end of each waveguide 10 a forms an inlet for that waveguide, andwherein the opposite end of each waveguide 10 a forms an outlet for thatwaveguide 10 a. The optical panel 10 also includes a light generationsystem 12, a housing 14 in which the light generation system 12 and theplurality of waveguides 10 a are mounted, and a coupler 16.

Each waveguide 10 a extends horizontally, and the plurality of stackedwaveguides 10 a extends vertically. The plurality of inlet ends definean inlet face 20 for receiving image light 22. The plurality of outletends define an outlet face 24 disposed substantially parallel with theinlet face 20 for displaying light 22. The light 22 may be displayed ina form such as, but not limited to, a video image 22 a.

The housing 14 is sized larger in height and width than the combinationof the light generation system 12 and the plurality of waveguides 10 a,to allow the placement of the plurality 10 a and light generation system12 therein. The housing 14 has an open front to allow for viewing of theoutlet face 24, and has a closed depth D looking from the open front tothe back of the housing 14.

The light generation system 12 provides the light viewed through thewaveguides 10 a. The light generation system 12 includes a light source30, and a light redirection element 32 that redirects incident light 22from the light source 30 into the coupler 16, which light redirectionelement 32, in combination with the coupler 16, allows for a reductionin the depth D of the housing 14. This reduction allowance occurs wherethe light redirection element 32 is configured for turning the light 22from a source 30, which source 30 is placed within the housing 14proximate to and parallel with the vertical stack of the plurality ofwaveguides 10 a, into the coupler 16, which then acutely turns the light22 into the waveguides 10 a. The coupler 16 is preferably effective forturning the image light in an exemplary range of about 45° up to about90°, in order to generate approximately horizontal transmission throughthe plurality of waveguides 10 a. The light generation system 12 mayalso include a modulator and further imaging optics. This lightgeneration system 12 is discussed with more particularity with respectto FIG. 2.

The parallel surfaces of the inlet face 20 and the outlet face 24 allowthe panel 10 and enclosing housing 14 to be made ultrathin in depth. Thepanel 10 has a nominal thickness T which is the depth of the waveguides10 a between the inlet face 20 and the outlet face 24, and thickness Tis substantially less than the height H and width W of the outlet face24. The panel 10 may be configured in typical television width to heightratios of 4:3 or 16:9, for example. For a height H of about 100 cm and awidth W of about 133 cm, the panel thickness T may be about 1 cm. Thedepth D may vary accordingly with the thickness T, but, in theembodiment described hereinabove, the depth D of the housing 14 ispreferably no greater than about 12 cm.

FIG. 2 is a side view cross sectional schematic of an optical panel 10.The panel 10 includes a plurality of stacked waveguides 10 a, a lightgeneration system 12, a coupler 16, and a housing 14.

The light generation system 12 includes a projector 60 which isoptically aligned with a light redirection element 32. An image isprojected onto the light redirection element 32, and is then redirectedto the coupler 16 for transmission through the waveguides 10 a fordisplay on the outlet face 24. Preferably, the projector 60 is disposedadjacent to the top of the inlet face 20 for projecting the image light22 generally parallel thereto, and is spaced therefrom a distancesufficient to allow for a turning of the image light 22 from the lightredirection element 32 into the coupler 16 for transmission through thewaveguides 10 a.

The projector 60 may include a suitable light source 30 for producingthe light 22. The light source 30 may be a light bulb (e.g. filament orarc type) or laser. The projector 60 may be a slide projector or videoprojector which may include a modulator 62 for modulating the light 22to form an image 22 a. The modulator 62 may be, for example, aconventional Liquid Crystal Display (LCD), a Digital Micromirror Device(DMD), a GLV, a laser raster scanner, a PDLC, an LCOS, a MEMS, or a CRT.The projector 60 may also include suitable image optics 64 fordistributing or broadcasting the image light 22 horizontally andvertically across the light redirection element 32 for properly focusedtransmission to the coupler 16. The image optics 64 may include focusingand expanding lenses and/or mirrors. One or more light generationsystems 12, such as between 2 and 4 such systems, may be used to providelight to one or more portions of the coupler 16. Expansion lenses may beused for both the imaging optics 64 and the light redirection element 32to expand the image light 22 both vertically and horizontally over thecoupler 16. Alternatively, suitable rastering systems may be used as thelight generation system 12 to form the image by rastering the imagelight 22 both horizontally and vertically across the coupler 16.

In the illustrated embodiment, the light 22 is initially projected fromthe projector 60 vertically downward inside the housing 14 to the bottomthereof where the light redirection elements 32 are mounted, and thelight redirection elements 32 then redirect the image light 22vertically upwardly at a small acute angle for broadcast over the entireexposed surface of the coupler 16. In an alternative embodiment, theprojector 60 could be placed beneath the inlet face 20 rather thanbehind the inlet face 20. In this alternative embodiment, the lightredirection elements 32 may be eliminated from the light generationsystem 12.

The allowable incidence angle of the image light 22 on the coupler 16 isdetermined by the capability of the coupler 16 to turn the light 22 intothe inlet face 20 of the panel 10. The greater the turning capability ofthe coupler 16, the closer the projector 60 may be mounted to thecoupler 16 for reducing the required depth D of the housing 14.

FIG. 3 is a schematic illustrating a horizontal and vertical crosssection of an optical panel 10. The panel 10 includes a plurality ofvertically stacked step optical waveguides 10 a of step index claddingtype (of the type shown in FIGS. 4 and 5 referenced above in the“Background of the Invention” section), a light generation system 12(see FIG. 2), a coupler 16, and a housing 14.

Each waveguide 10 a of the plurality of waveguides 10 a of step indexcladding type as shown in FIG. 3 includes a central transparent core 80having a first index of refraction. The core 80 may be formed of anymaterial known in the art to be suitable for passing light wavestherethrough, such as, but not limited to Plexiglas® or polymers.Plexiglas® is a brand of polymethylmethacrylate (PMMA) which can beobtained from Rohm+Haas in Philadelphia, Pa. The central core 80 mayalternatively be formed of an optical plastic, such as Lexan®,commercially available from the General Electric Company®, or glass,such as type BK7. Optical waveguides of this type, e.g. using individualglass sheets, are typically in the range of between 2 and 200 micronsthick and may be made of a manageable length and width. For lowresolution applications, such as for car brake or signal lights,indicator displays, or advertisement displays, the thickness of theglass sheets (or other material comprising the central core 80) may bemuch greater, such as ¼ inch or more. The central core 80 is laminatedbetween at least two cladding layers 82. The cladding layers 82immediately in contact with the glass have a second index of refractionwhich is discretely lower than that of the cores 80, thus allowing forinternal reflection of the light 22 as it is transmitted through thecores 80. The cladding 82 may be a suitable plastic, Plexiglas®, glass,adhesive, polyurethane, low refractive index polymer, or epoxy, forexample, and is preferably black in color. The manner of forming thecores 80 and cladding layers 82 and additional alternative compositionstherefor in step index cladding type optical waveguides are discussedwith more specificity in copending patent application Ser. No.09/469,092, filed Dec. 21, 1999, which application is herebyincorporated herein by reference thereto. Furthermore, additionalalternative configurations of the waveguides, e.g. using a blackpigmented layer in-between adjacent cladding layers of adjacent opticalwaveguides, may be implemented. These alternative configurations as wellas the various compositions/formation techniques of the black pigmentedlayer are also incorporated herein by reference to the copending patentapplication Ser. No. 09/469,092.

FIG. 4, as described above, is a side view cross sectional schematic ofa planar optical waveguide 10 a of step index cladding type. Thewaveguide 10 a includes core 80 having a central plane 91 associatedtherewith and is surrounded by discrete cladding layers 82. As shown inthe figure, light 22 traveling through the core is internally reflectedat the interfaces 95 between the core 80 and surrounding cladding layers82. At each of the reflections, or discrete bounces, there is a finiteamount of light loss.

The internal reflection occurs provided that the incident angle at eachinterface 91 is greater than θ_(t)=sin⁻¹(n_(o)/n_(f)), where n_(f) isthe refractive index of the core 80 and n_(o) is the refractive index ofthe cladding layer 82. If the waveguide 10 a has a thickness (i.e.including the core 80 and surrounding cladding layers 82) M and a lengthL, the path length p traversed by the light ray 22 will be expressed bythe following equations:

p=L/(cos θ_(t))

p=L/(1−sin² θ_(t))^(1/2)

p=L/(1−(n _(o) /n _(f))² sin² θ_(t))^(1/2)

p=n _(f) L/((n _(f) ² −n _(o) ²)sin² θ_(t))^(1/2)

The number of reflections, or bounces, N is then given by the followingequations:

N=(p/(M/sin θ_(t)))±1

N=((n _(o) L sin θ_(i))/(M(n _(f) ² −n _(o) ² sin² θ_(t))^(1/2)))±1

rounded off to the nearest whole number. The “±1”, which depends onwhere the light ray strikes the inlet face, is of no significance when Nis large, so it therefore may be ignored. For example, if M=50 μm,n_(f)=1.6, n_(o)=1.0, and θ_(I)=30°, N is approximately 6500 reflectionsper meter. Since a large number of reflections N usually occurs, thefinite amount of loss resulting from each discrete bounce at theinterfaces 95 is cumulative and thus becomes a significant factor inadversely affecting the efficiency of the optical panel. This undesiredquality, combined with the deleterious effects of chromatic dispersionas mentioned with respect to FIG. 5 above, are solved by using planaroptical waveguides having a gradient refractive index core as describedbelow with reference to FIGS. 6 and 7.

FIG. 6 is a side view cross sectional schematic of a planar opticalwaveguide 110 a that utilizes a gradient refractive index core 180, inaccordance with a preferred embodiment of the present invention. Theplanar optical waveguide 110 a comprises a planar sheet of gradientrefractive index core material 180 having a central plane 191 associatedtherewith. The core material 180 has an index of refraction whichdecreases as the distance from the central plane increases. Note thatthe solid horizontal lines having varying distances from one another inFIGS. 6 and 7 represent the gradient refractive index within the corematerial 180, i.e. not the shape of the waveguide. This decrease in therefractive index may occur gradually and continuously and at a constantrate. Alternatively, the rate of decrease in the refractive index mayincrease or decrease as the distance from the central plane increases.In other words, the rate of decrease in the refractive index may occurin an exponential fashion.

The composition of the core material 180 varies such that the corematerial 180 at the central plane 191 comprises a first material, andthe core material 180 at locations which are furthest perpendicularlyfrom the central plane 191 comprises a second material which isdifferent from the first material. At locations between the centralplane 191 and locations which are furthest perpendicularly from thecentral plane 191, the core material 180 comprises a mixture, orcombination, of the first and second material in varying amounts. Therefractive index of the first material is in the range of approximately1.40-3.00 and the refractive index of the second material is in therange of approximately 1.30-1.49. Preferably, the refractive index ofthe first material is in the range of approximately 1.49-1.56. In a morepreferred embodiment, the index of refraction of the first material is1.49 while the index of refraction of the second material is 1.33. Inanother preferred embodiment of the present invention, the index ofrefraction of the second material is in the range of approximately 87%to 99% of that of the first material, with 90% being the more preferredpercentage. In yet another preferred embodiment of the presentinvention, the second material may be comprised of a black material inorder to improve viewing contrast of the optical panel when viewing theoutlet face 24. This black gradient refractive index core material alsoabsorbs ambient light thereby further improving contrast. Thecomposition of the black material may be the same as for the blackpigmented layer described above. It is also understood that othermaterials which are dark in color may instead be employed within thesecond material.

General principles behind the fabrication of gradient index materialsare well known in the art. For example, gradient index lenses arecommercially available from Gradient Lens Corp. in Rochester, N.Y. andgradient index optical fibers are commercially available from Spec TranSpecialty Optics Company in Avon, Conn.

The first material preferably comprises polyacrylic or polycarbonate.Materials which may be used as the first material include, for example,Plexiglas®, silicone, silica, and diamond-like materials. Morepreferably, the first material comprises Plexiglas®. The second materialpreferably comprises a fluorinated polymer or polyacrylic. Materialswhich may be used as the second material include, for example, PMMA,polytetrafluoroethylene (Teflon®), silicone and silica. More preferably,the second material comprises PMMA or polytetrafluoroethylene).

As also illustrated in FIG. 6, light 22 traveling through the corematerial 180 is gradually and continuously refracted or bent towards thecentral plane 191, i.e. the location of the highest refractive index. Asshown in the figure, the light rays never strike the outside edge of thewaveguide 110 a where a reflection, or bounce, would take place. Thus,with the use of a gradient refractive index core 180, there are nodiscrete bounces of the light rays 22 and therefore little or no loss oflight occurs, thereby increasing the efficiency, performance and qualityof the optical panel.

FIG. 7 is a side view cross sectional schematic of the planar opticalwaveguide 110 a shown in FIG. 6 illustrating the reduction orelimination of chromatic dispersion, in accordance with a preferredembodiment of the present invention. Light 22 (i.e. comprising at least2 different wavelengths), entering the core material 180 at the sameangle, tends to separate due to the different indices of refraction.However, since the indices of refraction differ gradually andcontinuously, the separation that occurs of the light 22 (i.e. chromaticdispersion) is significantly restricted such that light separation ordisplacement of the individual light rays of the different wavelengthsdoes not occur or occurs only minimally upon exiting the core material180. Thus, with the use a gradient refractive index core 180, thedeleterious effects of chromatic dispersion usually exhibited by opticalwaveguides of step index cladding type is reduced or eliminated, therebyincreasing the performance and quality of the optical panel.

Similar to the reduction or elimination of chromatic dispersion thatoccurs when using the waveguides of the present invention, the light 22(i.e. whether composed of more than one wavelength or even onewavelength) exits the gradient refractive index core 180 in an exit coneangle within a range which is smaller than that exhibited by the stepindex cladding type waveguides.

As in the case of step index cladding type optical waveguides, thewaveguides 110 a of the present invention are in the form of flatribbons extending continuously in the horizontal direction along thewidth of the outlet face 24 (see FIG. 1). The ribbon waveguides 110 aare preferably stacked vertically along the height of the outlet face24. The vertical resolution of the panel 10 is thus dependent on thenumber of waveguides 110 a stacked along the height of the outlet face24. For example, a stacking of 525 waveguides would provide 525 verticallines of resolution for a standard television. A stacking of 1,100waveguides would similarly provide 1,100 vertical lines of resolutionfor a high definition TV (HDTV).

The plurality of waveguides 110 a may be stacked directly on top of oneanother or may optionally have a black pigmented layer between eachwaveguide 110 a for purposes of improving contrast and absorbing ambientlight. The black pigmented layer may be provided by way of, for example,spray deposition or adhesive. The black pigmented layer that may beemployed with the waveguides 110 a of the present invention may also beof the type mentioned in copending patent application Ser. No.09/469,092 mentioned above. The composition, manner of making, andutilization of the black pigmented layer is described in the Ser. No.09/469,092 application and is hereby incorporated herein by referencethereto.

Similarly, the light coupler 16 (see FIGS. 1-3) that may be employedwith the waveguides 110 a of the present invention may be of the typementioned in copending patent application Ser. No. 09/469,092 mentionedabove. The composition, manner of making, and utilization of the coupler16 is described in the Ser. No. 09/469,092 application and is herebyincorporated herein by reference thereto. The coupler is provided at theinlet face 20 to redirect light traveling from an axis non-perpendicularto the inlet face 20 to an axis perpendicular to the inlet face 20. Thecloser the refraction indices become between that of the first materialand second material, the more desirable it becomes to direct the lightcloser to an axis which is perpendicular to the inlet face 20. Forexample, when the index of refraction of the second material is in therange of approximately 97% to 99% of that of the first material, it ispreferable to provide light traveling from an axis which is within ±10°from a perpendicular to the inlet face 20.

The dimensions, i.e. the length, width, and thickness, of the waveguides110 a may be identical to the waveguides 110 a of step index claddingtype as described in the Ser. No. 09/469,092 application. Or, thedimensions may alternatively differ therefrom.

In a preferred embodiment, a light diffuser 117 (as shown in FIG. 8) maybe utilized at the outlet face 24. The diffuser 117 may be employed as asheet or film affixed to the ends of the waveguides 110 via heat bond,epoxy, adhesive, or may be embossed within the ends of the waveguides110 a at the outlet face 24 by, for example, compression of a die. Thediffuser 117 may alternatively be employed as a coating or layer whichis applied to the ends of the waveguides 110 via, for example, spraydeposition, painting, or various coating techniques.

In another preferred embodiment of the present invention, black materialmay be incorporated in the composition of the diffuser 117 as shown inFIG. 9. The black material may, for example, be of the type mentionedabove in connection with the second material within the gradientrefractive index core 180 within waveguide 110 a. It is also understoodthat other materials which are dark in color may instead be employed.The composition of the diffuser 117 varies such that the diffuser 117 atthe central plane 191 comprises a first material, and the diffuser 117at locations which are furthest perpendicularly from the central plane191 comprises a second material which is black in color. At locationsbetween the central plane 191 and locations which are furthestperpendicularly from the central plane 191, the diffuser 117 comprises amixture, or combination, of the first and second material in varyingamounts. The varying amounts may coincide or correspond with variancesof the refraction indices of the core material 180 as mentioned above.In other words, the concentration of the black (or dark) coloredmaterial increases as the distance from the central plane 191 increases.The increase in concentration may occur gradually and continuously.Alternatively, the increase in concentration may occur at a constantrate as the distance from the central plane 191 increases. Or, inanother alternative embodiment, the increase in concentration may occurat an exponential rate as the distance from the central plane 191increases. Note that the solid horizontal lines having varying distancesfrom one another in FIG. 9 represent the gradient black (or dark) colorwithin the diffuser 117, i.e. not the shape of the diffuser 117.

Those of ordinary skill in the art will recognize that manymodifications and variations of the present invention may beimplemented. For example, although the optical waveguides 110 a aredescribed in connection with an optical panel 10 of the configurationshown in FIG. 1, other optical panel types, such as that described inU.S. Pat. No. 5,625,736, may employ the waveguides 110 a of the presentinvention and can expect to have the same or similar advantagesassociated with the use thereof. The foregoing description and thefollowing claims are intended to cover all such modifications andvariations.

What is claimed is:
 1. An optical panel, comprising: a plurality ofstacked optical waveguides, each optical waveguide having a first endand a second end, wherein an outlet face is defined by the plurality offirst ends, and wherein an inlet face is defined by the plurality ofsecond ends, each optical waveguide comprising a planar sheet of corematerial having a central plane, the core material having an index ofrefraction which decreases as the distance from the central planeincreases, further comprising a diffuser provided at the outlet face. 2.The optical panel of claim 1, wherein the diffuser comprises a sheet orfilm.
 3. The optical panel of claim 1, wherein the diffuser comprises anembossed area at the outlet face.
 4. The optical panel of claim 1,wherein the diffuser comprises a coating or layer.
 5. The optical panelof claim 1, wherein the diffuser comprises diffusion material having adiffusion material central plane located at the first ends of eachwaveguide, each diffusion material central plane being coincident withthe central plane of the planar sheet of core material, the diffusionmaterial comprising a dark colored material, the concentration of thedark colored material increases as the distance from the diffusionmaterial central plane increases.
 6. The optical panel of claim 5,wherein the increase in concentration occurs gradually and continuously.7. The optical panel of claim 5, wherein the increase in concentrationoccurs at a constant rate as the distance from the diffusion materialcentral plane increases.
 8. The optical panel of claim 5, wherein theincrease in concentration occurs at an exponential rate as the distancefrom the diffusion material central plane increases.
 9. The opticalpanel of claim 1, wherein the diffuser comprises diffusion materialhaving a diffusion material central plane located at the first ends ofeach waveguide, each diffusion material central plane being coincidentwith the central plane of the planar sheet of core material, thediffusion material comprising a black colored material, theconcentration of the black colored material increases as the distancefrom the diffusion material central plane increases.
 10. The opticalpanel of claim 9, wherein the increase in concentration occurs graduallyand continuously.
 11. The optical panel of claim 9, wherein the increasein concentration occurs at a constant rate as the distance from thediffusion material central plane increases.
 12. The optical panel ofclaim 9, wherein the increase in concentration occurs at an exponentialrate as the distance from the diffusion material central planeincreases.
 13. A plurality of stacked planar optical waveguides for anoptical panel, each optical waveguide comprising: a planar sheet of corematerial having a central plane, the core material having an index ofrefraction which decreases as the distance from the central planeincreases, wherein the core material at a location which is furthestperpendicularly from the central plane is composed substantially of adark material.
 14. A plurality of stacked planar optical waveguides foran optical panel, each optical waveguide comprising: a planar sheet ofcore material having a central plane, the core material having an indexof refraction which decreases as the distance from the central planeincreases, wherein the core material at a location which is furthestperpendicularly from the central plane is composed substantially of ablack material.
 15. An optical panel, comprising: a plurality of stackedoptical waveguides, each optical waveguide having a first end and asecond end, wherein an outlet face is defined by the plurality of firstends, and wherein an inlet face is defined by the plurality of secondends, each optical waveguide comprising a planar sheet of core materialhaving a central plane, the core material having an index of refractionwhich decreases as the distance from the central plane increases,wherein the core material at a location which is furthestperpendicularly from the central plane is composed substantially of adark material.
 16. An optical panel, comprising: a plurality of stackedoptical waveguides, each optical waveguide having a first end and asecond end, wherein an outlet face is defined by the plurality of firstends, and wherein an inlet face is defined by the plurality of secondends, each optical waveguide comprising a planar sheet of core materialhaving a central plane, the core material having an index of refractionwhich decreases as the distance from the central plane increases,wherein the core material at a location which is furthestperpendicularly from the central plane is composed substantially of ablack material.